1. Field of the Invention
This invention relates to a tilt correcting optical system for use in a light beam scanning system which deflects a light beam such as a laser beam to scan a surface, and more particularly to a tilt correcting optical system which is used in a light beam scanning system, in which a light beam is deflected by a deflector such as a rotating polygonal mirror to scan a surface, and corrects fluctuation in the scanning line spacing generated due to surface tilt in the deflector.
2. Description of the Related Art
There have been well known light beam scanning systems in which a light beam is deflected by a deflector such as a rotating polygonal mirror or a galvanometer mirror to scan a surface.
In such light beam scanning systems, there has been a problem that the position of the scanning spot fluctuates in the sub-scanning direction (a direction normal to the main scanning direction) on the surface to be scanned, which results in fluctuation in the scanning line spacing. In the case of a rotating polygonal mirror, error in parallelism of each reflecting surface relative to the rotation axis due to manufacturing accuracy causes the phenomenon, and in the case of a galvanometer mirror, wobbling of the mirror causes the phenomenon. In this specification, the error in parallelism of the reflecting surfaces and the wobbling of the galvanometer mirror will be referred to as "surface tilt", hereinbelow.
It has been known that the surface tilt can be compensated for by use of a tilt correcting optical system comprising a positive cylindrical lens or a cylindrical mirror.
However recently, scanning with a higher accuracy at a higher density has come to be required and compensation for the surface tilt must be effected with a higher accuracy. It is difficult to compensate for the surface tilt with a higher accuracy just with a cylindrical lens or a cylindrical mirror.
Thus there has been proposed in Japanese Patent Publication No. 4(1992)-21164 a tilt correcting optical system comprising a combination of a negative cylindrical lens and a positive cylindrical mirror. An example of such a tilt correcting optical system will be described with reference to FIGS. 5 and 6, hereinbelow.
In FIGS. 5 and 6, a light beam L emitted from a laser 111 passes through a cylindrical lens 112 and is focused on a reflecting surface 114 of a rotating polygonal mirror 113 as a line image perpendicular to the axis of rotation of the polygonal mirror 113. As the polygonal mirror 113 rotates in the direction of arrow R, the light beam L is deflected. FIG. 5 shows the optical path of the deflected light beam L as seen in a direction parallel to the axis of rotation of the polygonal mirror 113 and FIG. 6 shows the same as seen in a direction perpendicular to the axis of rotation of the polygonal mirror 113. The main scanning of the deflected light beam L will be described first with reference to FIG. 5, hereinbelow. The light beam L reflected and deflected by the reflecting surface 114 of the rotating polygonal mirror 113 enters a scanning lens 115. The parallel light beam L further passes through a negative cylindrical lens 116 and impinges upon a positive cylindrical mirror 117 to be converged on a surface to be scanned 120. Thus the light beam L is caused to repeatedly scan (the main scanning) the surface 120 in the direction of arrow X. The negative cylindrical lens 116 and the positive cylindrical mirror 117 disposed between the scanning lens 115 and the surface 120 to extend in parallel to the main scanning direction respectively diverge and converge the light beam L only in a direction perpendicular to the main scanning direction (the sub-scanning direction), and accordingly the light beam L just passes through the negative cylindrical lens 116 and is just reflected by the positive cylindrical mirror 117 as seen in FIG. 5. As described above, the rotating polygonal mirror 113 is apt to generate surface tilt and how the surface tilt is compensated for will be described hereinbelow with reference to FIG. 6.
The light beam L reflected and deflected by the reflecting surface 114 of the rotating polygonal mirror 113 is somewhat diverged by the scanning lens 115. Then the diverged light beam L impinges upon the positive cylindrical mirror 117 which converges the light beam L only in the sub-scanning direction (the direction of arrow Y) on the surface 120 to be scanned. At this time, the cylindrical mirror 117 converges light reflected by a point on the reflecting surface 114 in the same position on the surface 120 as seen along the sub-scanning direction irrespective of the surface tilt of the polygonal mirror 113. That is, even if the optical path of the light beam L reflected by the polygonal mirror 113 deviates in the vertical direction as seen in FIG. 6, the deviation can be compensated for by the cylindrical mirror 117. The cylindrical lens 117 between the scanning lens 115 and the cylindrical mirror 117 corrects the curvature of field.
With such a tilt correcting optical system, the surface tilt can be compensated for at a higher accuracy than when the surface tilt is compensated for just by a positive cylindrical lens or a cylindrical mirror. However the arrangement of the tilt correcting optical system described above increases the number of parts, which leads to increase in the cost and at the same time leads to increase in the accumulated error in assembly of the parts.